This invention relates to a method of calibrating vehicle speed signals.
It has become increasingly common for modern vehicles to be equipped with electronic speed sensors that generate an electromagnetic signal which varies in response to the speed of a corresponding wheel. These wheel speed signals are used, for example, in adaptive braking and/or adaptive traction systems to effect electronic control of the vehicle's brakes to prevent skidding and to transfer power from one wheel to the other if one wheel loses traction on the driving surface. Wheel speed signals may also be used to drive an electronic speedometer.
In general, wheel speed sensors consist of an electromagnetic pickup mounted adjacent a toothed tone wheel which is mounted for rotation with the corresponding vehicle wheel. The change in magnetic reluctance due to the constantly changing magnetic path depending upon whether a tooth or a space is moving across the electromagnetic pickup induces a pulsed output from the electromagnetic pickup. This output signal is transmitted through a calibration circuit which generates a square wave output signal from the input signal generated by the electromagnetic pickup. The frequency of this square wave signal is a function of instantaneous wheel speed. The automotive industry has adopted a standard for electronic speedometers and other devices in which such devices are designed to receive a nominal number of pulse counts per unit of distance traveled by the vehicle. The electronic speedometer is designed to indicate speed as a function of the frequency of this signal. However, the number of pulses generated by any typical speed sensing unit per unit of distance traveled can vary substantially, depending upon the size of the tire, diameter and number of teeth in the tone wheel, and tire inflation. Accordingly, a calibration factor must be used to convert the actual pulse counts per unit of distance traveled by the wheel into a signal that the electronic speedometer is designed to display that accurately reflects vehicle speed.
Determination of this calibration factor has proven to be in practice quite difficult. Of course, the correction factor can be calculated from known information, including the number of teeth on the tone wheel, rolling radius of the tire, etc., but this calculation is only approximate at best, and cannot take into account varying factors, such as tire inflation. Furthermore, a change in tire size will affect the calibration factor, and since changes in tire size are not recognized by the vehicle operator as affecting the speedometer calibration, it is doubtful that recalibration would be effected. Another way of establishing a calibration factor is to count the pulses as the vehicle is driven, calculate the calibration factor, and then set dip switches in the electronic hardware to set the new calibration factor. Obviously, this method also has disadvantages and is not likely to be performed on a regular basis.